I don't know the op or Rik and I don't have a dog in this fight, but the most disappointing thing about this thread is that "no answer" is "an answer"

race boat and or team u joint should not slow down a ace mechanic tech

Replacing a critical part that has failed for an unknown reason makes little sense.

PSA

If you have the arneson 7M take the side cover off the standoff and feel the washers under the nut that holds the drive to the standoff box and make sure they do not spin, and feel tight. We had a drive that came loose and after inspecting it we found that with the lock nut tightened down on the stud until it runs out of thread and the stud tightened into the drive till it runs out of thread that there is .570" under washer to face of drive. The thickness of the standoff box on that hole was .364". So even if the nut is tightened as tight as you can get it the washer is not touching. The thickness of the standoff box holes were around .364" thick where the bottom holes are, the next holes up were .415" ,next ones up were .477" the top ones were .515" thick. A couple of the studs did have shorter flat sections so they might have been able to tighten up just on the top holes.

I had another person check their drives and four of the washers were loose on one drive and three on the other after they tried to tighten them as tight as possible. It's a simple fix just a thicker washer. But if you have these drives it might be something to check. When your tightening the nut make sure the washer gets tight and will not spin before the nut gets real tight then it should be good if you can get at least another turn out of it.

I have had Arnesons since 2002 and have broke several props (29), several transmissions(11), and a lot of driveshafts (7) from power but have never broke one of the arnesons. And I have taken those same arnesons off that boat and Rik rebuilt them with new propshafts since they were beat up from the props breaking and that is what I am putting on my new boat so this post is not against Arneson just something we have found wrong with a set that should probably be checked if you have them.

I admire the way our little "band of brothers" is always happy to share information to help one another out. This is a great example of that! The burning question in my mind is, have you ever shared this info with Rik?

Rik does seem to be extremely helpful... to those willing to pay astro-f#cking-nomical prices for his products. Everything will work awesome until you have them. Everyone forgets you need another 10k to fine tune props

I just started measuring it this afternoon, since I made the post I measured the other side and the standoff box is around .120" thicker consistently. So only the bottom washers were not tight.

You're not the first guy that has had a rough time getting responses after you fork out your first born for arnesons

I got good news Rik called me today. He said he should have my gearing by Friday and back togethers next week. Thanks everyone for all the help!

Hoping that you paid attention to what Tyson was referencing in his post, as in check then double check everything before you run them. The differences the he found may not sound substantial, but it's amazing how fast being .050 loose can become .125, then .250 and then vibrate itself into pieces...